The present invention relates to a device for measuring radius of curvature, more particularly, a device for measuring and testing radius of curvature of an optical lens and a spherical mirror and a method thereof.
Such a device for measuring radius of curvature comprises, as shown in FIG. 8, a laser source which is so arranged that collimated light beam 3 having a small radius is illuminated on a surface 2 on which the radius of a substance is measured. The substance to be measured moves in the tangential direction at the measuring point on the surface at which the curvature is to be measured, through an infinitesimal distance by a driving device (not shown), for example, the driving device such as a micromoter. A projection plate 4 is arranged between the laser light source 1 and the substance to be measured in parallel with a virtual osculating plane at the measuring point of the measuring surface so that the projecting position of the laser beam reflected on the measuring point can be designated.
In the apparatus thus constructed, the curvature of the measuring point is obtained as follows. The substance to be measured is, in the first place, moved and the driving device is adjusted in such a manner that the centered or illuminated laser beam 3 is again returned in the same position, thereby incident angle .alpha. becoming zero. This zero incident angle .alpha. is shown in FIG. 8 by a broken line 2. The substance to be measured is displaced in the tangential direction at the measuring point by the infinitesimal distance d. The position of the measuring surface thus displaced is shown in FIG. 8 by a solid line 2'. The reflection direction of the reflected laser beam is changed from 5 to 5' by such a displcement so that the projected position is also displaced by the infinitesimal distance x. Supposing that the incident angle of the laser beam 3 to the measuring surface 2' is .theta. after displacement, the distance between the projection plate 4 and the osculating plane of the measuring surfaces 2, 2' is l the radius of curvature is .gamma., and x, d&lt;l, .gamma., .theta. may be made very small, and proximated to .theta..about.sin .theta..about.tan .theta.. Therefore, following relations are obtained. EQU x.about.l.multidot.2.theta. (1) EQU .gamma..theta..about.d (2)
From these equations (1) and (2), is removed, the radius of curvature .gamma. is obtained as follows. EQU .gamma.=2dl/x (3)
In the actual measurement, the displacement position d of the substance to be measured by which a constant value x may be obtained, is preferably measured. In this way, the accuracy of measurement may be increased. For example, when a slit is provided at the predetermined position of the projection plate and a photo detector is arranged behind the slit, the laser beam reflected from the measuring position is detected through the slit in case of displacing the substance to be measured and thus the displacement distance of the subtance to be measured may be obtained.
In the above conventional device for measuring radius of curvature, the measuring precision of radius of curvature is determined by the measuring resolution of x and d, so that the smaller the radius of curvature the smaller the value of d and thus the accuracy of measurement becomes decreased relatively. Moreover, the radius of curvature can not be measured for the substance having a thick rear face.